1. Field of the Invention
The present invention relates to a speaker system, and more particularly to a phase-inverted type of speaker system.
2. Description of the Related Art
Widely known is a phase-inverted type of speaker system (which will be hereinafter referred to as a bass-reflex type of speaker system) having a duct for giving a suitable change in phase to a sound wave radiated from a rear surface of a speaker unit and a radiating sound wave in phase with a sound wave radiated from a front surface of the speaker unit.
Such a bass-reflex type of speaker system has a structure as shown in FIGS. 1a and 1b, wherein FIG. 1a is a front elevation of the speaker system, and FIG. 1b is a cross section taken along the line X--X in FIG. 1a. The speaker system includes an enclosure 1, a baffle plate 2 forming a front surface of the enclosure 1, a speaker 3, and a pair of bass-reflex ducts (which will be hereinafter referred to simply as ducts) 4. The speaker unit 3 is mounted to the baffle plate 2. Each duct 4 is cylindrical, and it is integrally formed with the baffle plate 2 or is mounted to the baffle plate 2.
The phase of sound wave A.sub.2 radiated from the rear surface of the speaker unit 3 is opposite to the phase of sound wave A.sub.1 radiated from the front surface of the speaker unit 3. The sound wave A.sub.2 radiated to an inside space 5 of the enclosure 1 is introduced into the duct 4 from an inner opening 4a of the duct 4, and is then radiated from an outer opening 4b of the duct 4 to the front side of the speaker system. The volume of the inside space 5, the length of the duct 4, the areas of the inner and outer openings 4a and 4b, etc. are set so that the phase of the sound wave A.sub.2 to be radiated from the outer opening 4b becomes the same as the phase of the sound wave A.sub.1.
However, the bass-reflex type of speaker system as shown in FIGS. 1a and 1b has the following problems.
First, there is a problem in production. As the duct is formed usually in a cylindrical or prismatic shape, a large space for packaging is required in packaging a plurality of such ducts as independent parts, thus greatly reducing a transporting efficiency and increasing a transporting cost.
That is, as the duct is cylindrical or prismatic, a plurality of such ducts cannot be stacked together in a telescopic fashion. In the condition where the duct is mounted to the baffle plate, a plurality of such baffle plates cannot be stacked in close relationship to each other. Accordingly, a depth corresponding to the total length of the ducts is required in packaging the baffle plates, thus resulting in a large dead space in a packaged condition of the baffle plates.
Secondly, there is a problem in sound quality. As shown by an arrow M in FIG. 2, resonance in an air column is possibly generated in the duct 4. At a frequency of a sound wave having a wavelength four times the length of the duct 4, the resonance shown by the arrow M is generated to adversely affect a sound quality.
Further, when the area of the inner opening 4a of the duct 4 is small, a so-called wind noise is generated by air flowing from the inner opening 4a into the duct 4, which also causes a deterioration in sound quality. Accordingly, it is necessary to ensure a sufficient area of the inner opening 4a to such an extent as to suppress the wind noise, which will reduce a degree of freedom in designing the shape of the duct 4.
Further, in the speaker system having a plurality of, e.g., a pair of ducts 4 as shown in FIGS. 1a and 1b, a slight phase difference between sound waves to be radiated from the ducts is apt to generate, which also causes a deterioration in sound quality.